1. Field of the Invention
This invention relates to an apparatus and method for friction stir welding, and, in one aspect, to an apparatus and method for friction stir welding using filler material.
2. Description of the Related Art
Workpieces made of some commercial metallic alloys (e.g., most 2000- and 7000-series aluminum alloys) are difficult to join by conventional welding processes (e.g., arc welding processes). For example, 2000-series aluminum alloys are sensitive in heat-affected zones (HAZs) where the base metal reaches temperatures between liquidus and solidus during welding. In this area, partial melting at grain boundaries forms a network containing brittle intermetallic compounds of CuAl2. Thus, weld ductility can be substantially reduced. Further, conventional welding processes can create geometric distortions near the weld joint due to high temperature gradients (non-uniform heating) induced in the workpiece during welding. These distortions can cause warping and other dimensional defects in the workpiece, as well as residual stresses that can lead to premature failure by cracking in the weldment or HAZ (either due to static and/or fatigue stresses), lamellar tearing, or by stress-corrosion cracking in some metals.
In addition, some alloys and types of weld joints are difficult to join except in a flat position. For example, thick weldments are typically made in the flat position unless some way is provided to retain the weld metal in the joint, such as with chilled backing plates and to quickly solidify the weld metal and/or with chilled shoes to hold the weld metal in the joint during solidification. Further, traditional welding processes produce welding fumes, spatter, and a possibility of porosity in the deposited weld metal, due to entrained gases. Yet further, certain metals (e.g., aluminum and aluminum alloys) can have surface oxide layers that are insoluble in the molten weld metal. Thus, these oxide layers can readily become entrained in the weldment, causing defects that can decrease static and fatigue strength of the weldment. Accordingly, the oxide layers are typically removed by pickling, grinding, and/or brushing prior to the workpieces being conventionally welded.
Friction stir welding processes can overcome many of the problems encountered with traditional welding processes in metals. In a typical friction stir welding process, illustrated in FIG. 1, a cylindrical tool 102 having a shoulder 104 and a pin 106 is rotated (as indicated by arrow 107) and plunged (as indicated by arrow 109) into a joint line 108 between two abutted workpieces 110, 112 of sheet or plate material. As the pin 106 contacts the workpieces 110, 112, friction between the pin 106 and the workpieces 110, 112 generates heat to plasticize an area of the workpieces 110, 112 adjacent the joint line 108. As the pin 106 continues to plunge into the workpieces 110, 112, more material is plasticized, thus allowing the pin 106 to plunge further into the workpieces 110, 112. Plunging stops when the shoulder 104 comes into contact with and is forced against the workpieces 110, 112. Each of the workpieces 110, 112 is clamped onto an anvil 114 in such a manner as to prevent the abutting joint faces of the workpieces 110, 112 from being forced apart.
Frictional heat is generated between the shoulder 104, the pin 106, and the workpieces 110, 112. This heat causes the workpieces 110, 112 to soften or plasticize without reaching their melting point and allows the tool 102 to traverse (as indicated by arrow 113) along the joint line 108. As downward pressure is maintained (as indicated by the arrow 109) and the tool 102 moves along the joint 108 (as indicated by the arrow 113), the plasticized material is transferred from the leading edge 116 of the tool 102 to the trailing edge 118 of the tool 102, and is forged by intimate contact with the shoulder 104 and the pin 106, and is forced against the anvil 114. A solid-phase bond 120 is left between the workpieces 110, 112.
Process advantages can result from such a friction stir welding process (as in generally all friction welding processes) taking place in a solid phase below melting points of the materials being joined. Thus, since no melting occurs, continuous networks of intermetallic compounds (e.g., intermetallic compounds of CuAl2 in 2000-series aluminum alloys) have little opportunity to form and generally no fumes or spatter are created. The friction stir welding process also produces lower distortion in the workpieces 110, 112, since much less heat is transferred into the workpieces 110, 112. Further, the friction stir welding process can be performed in any position, since the material along the joint line 108 is plasticized, not melted, and readily remains in place. Yet further, surface oxide layers are generally swept away during the friction stir welding process due to the friction between the shoulder 104, the pin 106 and the workpieces 110, 112. Thus, pickling, grinding, and/or brushing of the workpieces 110,112 are not generally required.
The friction stir welding process has several limitations, however. First, joints between workpieces to be friction stir welded generally must have better fit up than that required for joints between workpieces that are conventionally welded. In general, any gap between the workpieces to be joined must be less than 10 percent of the thickness of the workpieces. For example, if the workpieces to be joined have thicknesses of 12.7 mm (0.5 in), the maximum generally-acceptable gap is 1.27 mm (0.05 in). This requirement is due in large part to the fact that known friction stir welding processes do not employ the use of filler materials, which can be used to fill excessive gaps in weld joints. Such stringent fit up requirements can result in higher workpiece preparation costs and workpiece fixturing costs. These costs can escalate dramatically when large workpieces are joined. Further, traditional friction stir welding processes can generally be used on only a limited number of joint types, e.g., butt joints and edge joints. Joint types requiring a fillet, e.g., corner joints, lap joints, and filleted T-joints, cannot generally be accomplished using traditional friction stir welding processes, as filler metal is required to produce the fillet.
The present invention is directed to overcoming, or at least reducing, the effects of one or more of the problems set forth above.
In one aspect of the present invention, a friction stir welding tool includes a body having passageway therethrough through which a filler material may pass and a pin extending from a bottom of the body capable of creating friction when rotated against a workpiece to weld the workpiece.
According to another aspect of the present invention, an apparatus capable of friction stir welding is provided. The apparatus includes a friction stir welding tool having a body having an upper surface defining an entrance opening and a lower surface, and a pin having a lower surface defining an exit opening, wherein the pin extends from the lower surface of the body. The friction stir welding tool further includes a passageway from the entrance opening to the exit opening and is capable of allowing a filler material to pass therethrough. The apparatus further includes a spindle capable of rotating the friction stir welding tool, wherein the spindle has a passageway therethrough capable of allowing the filler material to pass therethrough and wherein the passageway through the spindle communicates with the passageway through the friction stir welding tool. Further, a filler material feeder is provided that is capable of feeding the filler material, wherein the filler material feeder feeds the filler material into the passageway through the spindle, and a device capable of holding a workpiece.
In another aspect of the present invention, a friction stir welding method includes applying a frictional heating source to a workpiece to plasticize a volume of the workpiece and applying the frictional heating source to a filler material to plasticize the filler material. The method further includes introducing the filler material into the volume of the workpiece and incorporating the filler material into the volume of the workpiece.